Silica Sol

ABSTRACT

The present invention relates to a silica sol having a pH above about 10, molar ratio of Si02:M20, in which M is alkali metal, of from about 6:1 to about 16:1, concentration of soluble silica above about 3000 mg Si02/l, and S-value of from about 18 to about 40%. The invention further relates to a process for producing a silica sol which comprises acidifying an aqueous alkali metal silicate solution to a pH of from 1 to 4 to form an acid sol, alkalizing the acid sol by addition of aqueous alkali metal silicate solution to obtain an intermediate sol and interrupt the addition of aqueous alkali metal silicate solution when the intermediate sol has reached a pH of from about 5 to about 8, stirring the intermediate sol for a time period of from about 10 to about 6000 seconds without addition of aqueous alkali metal silicate solution, alkalizing the intermediate sol by addition of aqueous alkali metal silicate solution to obtain a silica sol having a pH above about 10 and molar ratio of Si02:M20, in which M is alkali metal, of from about 6:1 to about 16:1, and optionally adding an aluminum compound to the acid sol, intermediate sol or silica sol. The invention further relates to a silica sol obtainable by the process of the invention. The invention further relates to the use of the silica sol as a flocculating agent, in water purification and in producing paper and board. The invention further relates to a process for producing paper and board which comprises: (i) providing an aqueous suspension comprising cellulosic fibers; (ii) adding to the suspension one or more drainage and retention aids comprising the silica sol; and (iii) dewatering the obtained suspension to provide a sheet or web of paper or board.

FIELD OF THE INVENTION

The present invention relates to a silica sol and its production anduse. The invention provides a silica sol which is suitable for use as aflocculating agent, in particular as a drainage and retention aid inpapermaking.

BACKGROUND OF THE INVENTION

In the papermaking art, an aqueous suspension containing cellulosicfibers, and optional filler and additives, is fed into a headbox whichejects the cellulosic suspension onto a forming wire. Water is drainedfrom the cellulosic suspension to provide a wet paper web which isfurther dewatered and dried in the drying section of the paper machine.Drainage and retention aids are conventionally introduced in thecellulosic suspension to facilitate drainage and increase adsorption offine particles onto the cellulosic fibers so that they are retained withthe fibers.

Silica sols are widely used as drainage and retention aids, usually incombination with charged organic polymers. Such additive systems areamong the most efficient now in use in the papermaking industry.Examples of silica sols known in the art include those disclosed in U.S.Pat. Nos. 4,388,150 and 6,372,806 as well as International Patent Appl'nPubl. Nos. WO 91/07350, 91/07351, 94/05596, 98/30753, 98/56715,00/66491, 00/66492, 2005/097678, 2005/100241, 2008/150230 and2010/006994.

It would be desirable to provide a silica sol with improved drainage andretention performance in the production of paper and board. It wouldalso be desirable to provide a method for producing such a silica sol.It would also be desirable to provide a process for making paper andboard with improved drainage and retention performance.

SUMMARY OF THE INVENTION

The present invention is generally directed to a silica sol having a pHabove about 10, molar ratio of SiO₂:M₂O, in which M is alkali metal, offrom about 6 to about 16, concentration of soluble silica above about3000 mg SiO₂/l, and S-value of from about 18 to about 40%.

The invention is further directed to a process for the production of asilica sol which comprises:

-   (a) acidifying an aqueous alkali metal silicate solution to a pH of    from 1 to 4 to form an acid sol,-   (b) alkalizing the acid sol by addition of aqueous alkali metal    silicate solution to obtain an intermediate sol and interrupt the    addition of aqueous alkali metal silicate solution when the    intermediate sol has reached a pH of from about 5 to about 8,-   (c) stirring the intermediate sol for a time period of from about 10    to about 6000 seconds without addition of aqueous alkali metal    silicate solution,-   (d) alkalizing the intermediate sol by addition of aqueous alkali    metal silicate solution to obtain a silica sol having a pH above    about 10 and molar ratio of SiO₂:M₂O, in which M is alkali metal, of    from about 6:1 to about 16:1, and-   (e) optionally adding an aluminum compound to the acid sol,    intermediate sol or silica sol.

The invention is also directed to a silica sol obtainable by the processof the invention.

The invention is further directed to the use of the silica sol of theinvention as a flocculating agent, both in water purification and as adrainage and retention aid in the production of paper and board.

The invention is further generally directed to a process for producingpaper and board which comprises:

-   -   (i) providing an aqueous suspension comprising cellulosic        fibers;    -   (ii) adding to the suspension one or more drainage and retention        aids comprising a silica sol of the invention; and    -   (iii) dewatering the obtained suspension to provide a sheet or        web of paper or board.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention there is provided a silica sol,herein also referred to as the “sol”, “sol of silica particles”, or “solcontaining silica particles”, which is suitable for use as aflocculating agent, e.g. in papermaking and water purification, inparticular as a drainage and retention aid in papermaking. The term“drainage and retention aid”, as used herein, refers to one or moreadditives which, when added to an aqueous cellulosic suspension, givebetter drainage and/or retention than what is obtained when not addingthe said one or more additives.

The silica sol of the invention provides several improvements when usedas an additive in the production of paper and board, in particular whenproducing paper and board from cellulosic suspensions having lowconductivity and/or cationic demand. Such cellulosic suspensions arecommonly used in the production of paper and board from chemical pulpand recycled fiber pulp, e.g. in non-integrated paper and board millsand mills using a substantial amount of recycled fiber. The use of thesilica sol according to the invention makes it possible to increasedrainage (dewatering) and retention, decrease the steam consumption inthe drying section and/or increase the speed of the paper and boardmaking machine, to use a lower dosage of additive to give acorresponding drainage and retention effect, and improve paper and boardformation, thereby leading to improved paper and board making processes,improved quality of the paper and board produced, and economic benefits.The silica sol of the invention also exhibits improved stability, inparticular when being diluted with water, and improved stability towardsprecipitation, aggregation and gelation. Hereby the present inventionprovides benefits in terms of application and use of the silica sol.

Preferably, the sol according to the invention is aqueous. The solcontains silica particles, i.e. particles of or based on silica or SiO₂,which are preferably anionic and preferably colloidal, i.e., in thecolloidal range of particle size. Aqueous dispersions of this type areusually referred to as sols. Preferably, the silica particles have beenprepared by condensation polymerisation of siliceous compounds, e.g.silicic acids and silicates. Other elements or components may also bepresent in the aqueous phase of the sol and/or in the silica particles.Such elements or components may be present as impurities or as a resultof deliberate introduction or modification.

The silica sol of to the invention has a pH of at least about 10.0, orat least about 10.2 and usually at least about 10.5. Usually, the pH ofthe silica sol is up to about 11.5, more often up to about 11.3 andoften around about 11.0

The silica sol of the invention has a molar ratio of SiO₂:M₂O, in whichM is alkali metal, of at least about 6:1, suitably from at least about8:1, preferably at least about 10:1. The molar ratio of SiO₂:M₂O isusually up to 16:1, suitably up to about 15:1 or up to about 14:1. Thealkali metal M can be lithium, sodium, potassium and a mixture thereof,preferably sodium, and the molar ratios defined above are preferably themolar ratio of SiO₂:Na₂O.

The silica sol of the invention has a concentration of soluble silica ofat least about 3,000 mg SiO₂ per litre of liquid sol (mg SiO₂/l),usually at least about 3,500 mg SiO₂/l or at least about 4,000 mgSiO₂/l. The concentration of soluble silica may be up to about 12,000 mgSiO₂/l, usually up to about 10,000 mg SiO₂/l or up to about 8,000 mgSiO₂/l, suitably up to about 6,000 mg SiO₂/l. The silica sol ispreferably aqueous, and the above values are preferably per litre ofaqueous sol (mg SiO₂/l). The concentration of soluble silica is measuredand calculated as described by J. B. Mullin and J. P. Riley in Anal.Chim. Acta, vol. 12 (1955), pp. 162-176 using a UV-VIS SpectrophotometerHelios Alfa, Unicam

The silica sol of the invention usually has a conductivity of at leastabout 3.0 mS/cm or at least about 4.0 mS/cm, suitably at least about 5.0or at least about 6.0 mS/cm, preferably at least about 7.0 mS/cm.Usually, the conductivity is up to about 12.0 mS/cm, suitably up toabout 10.0 mS/cm. The conductivity can be measured by means of knowntechnique, e.g. using a Conductivity Meter CDM92, Radiometer,Copenhagen.

The silica sol of the invention usually has an S-value of at least about18% or at least about 20%, suitably at least about 26% or at least about30%. Usually, the S-value is up to about 40% or up to about 38% andsuitably up to about 35%. The S-value is measured and calculated asdescribed by R. K. Iler & R. L Dalton in J. Phys. Chem. 60(1956),955-957. The S-value of a silica sol indicates the degree of aggregateor microgel formation and a lower S-value indicates a higher degree ofaggregate or microgel formation.

The silica sol of the invention may comprise aluminum, and the silicasol may be modified with aluminum. The aluminum may be present in thecontinuous, or aqueous, phase of the sol and/or in the silica particles.Suitably, the silica sol contains silica particles which are modifiedwith aluminum, and preferably the silica particles are surface-modifiedwith aluminum. If the silica sol comprises aluminum, and if the soland/or the silica particles are modified with aluminum as defined above,the silica sol usually has a molar ratio of Si:Al of at least about 4:1or at least about 25:1, suitably at least about 50:1, preferably atleast 75:1, and the molar ratio of Si:Al may be up to about 400:1usually up to about 250:1, suitably up to about 200:1 and preferably upto about 150:1.

Alternatively, the silica sol of the invention may be free oressentially free from aluminum, and the silica sol may contain silicaparticles which are not modified with aluminum. This means that aluminummay be present in the sol as an impurity, in the continuous, or aqueous,phase of the sol and/or in the silica particles, and then the silica solusually has a molar ratio of Si:Al of at least about 400:1 or at leastabout 500:1, suitably at least about 600:1, depending on the propertiesof the starting material used to prepare the silica sol.

The silica particles of the sol usually have a specific surface area ofat least about 300 m²/g or at least about 500 m²/g, suitably at leastabout 600 m²/g and preferably at least about 750 m²/g. The specificsurface area is usually up to 1500 m²/g or at least about 1300 m²/g,suitably up to about 1100 m²/g. The specific surface area is measured bymeans of titration with NaOH as described by G. W. Sears, Jr. inAnalytical Chemistry 28(1956):12, 1981-1983, after appropriate removalof or adjustment for any compounds present in the sample that maydisturb the titration like aluminum and boron compounds, for example asdescribed by Sears and in U.S. Pat. No. 5,176,891. The specific surfaceareas given herein represent the average specific surface area of thesilica particles present in the sol.

The silica sol of the invention usually has a silica (SiO₂) content ofat least about 2% by weight or at least 3% by weight, suitably at leastabout 5% by weight or at least about 8% by weight and preferably atleast about 10% by weight. Usually, the silica content is up to about30% by weight or up to about 20% by weight and suitably up to about 15%by weight. In order to simplify shipping and reduce transportationcosts, it is generally preferable to ship a high concentration silicasol of the invention but it is also possible to dilute and mix thesilica sol with water to a substantially lower silica content prior touse, e.g. a silica content of from about 0.05 to about 2% by weight orfrom about 0.1 to 1.5% by weight. Dilution and mixing with water priorto use in a process for producing paper and board may result in goodmixing with the furnish components of the cellulosic suspension. Oneadvantage associated with the silica sol of the invention is that it isless sensitive towards dilution with water, in particular watercontaining a substantial amount of calcium, which is commonly used inthe papermaking industry, and hereby the present silica sol showsimproved stability towards precipitation, also during prolonged storageat elevated temperatures, which may lead to aggregation and gelation.

Another advantage associated the present silica sol is that it may beadded as is, or at a high silica content, to the cellulosic suspension,i.e. without dilution and mixing with water, and still results in goodor improved mixing with the furnish components of the cellulosicsuspension.

The viscosity of the silica sol of the invention can vary depending on,for example, the silica content of the sol. The viscosity is often atleast about 2 cP, or at least about 3 cP, suitably at least 3.5 cP.Usually, the viscosity is up to about 15 cP or up to about 10 cP,suitably up to about 8.0 cP. The viscosity can be measured by means ofknown technique, e.g. using a Brookfield Viscometer LVDV-II+Pro, ULA(00) spindle.

According to the invention, the silica sol can be produced using analkali metal silicate as a starting material by a process comprisingacidification, alkalization, particle formation, particle growth,particle aggregation and microgel formation, optional aluminummodification and optional concentration. Preferably, the startingmaterial is an aqueous solution of alkali metal silicate, or waterglass. Examples of suitable alkali metal silicates include lithium,sodium, potassium silicates and mixtures thereof, preferably sodiumsilicate. The alkali metal silicate usually has a molar ratio ofSiO₂:M₂O, in which M is alkali metal, of from about 1:1 to about 15:1,suitably from about 1.5:1 to about 4.5:1, preferably from about 2.5:1 toabout 4.0:1. The aqueous alkali metal silicate used usually has a SiO₂content of from about 1 to about 35% by weight, suitably from about 3 toabout 30% by weight. The aqueous alkali metal silicate solution usuallyhas a pH above about 12, typically above about 13.

In the process, the aqueous alkali metal silicate solution is subjectedto acidification. During the acidification, silica particle formationand growth preferably take place. The acidification can be carried outby addition of a mineral acid, e.g. sulfuric acid, hydrochloric acid andphosphoric acid, and/or by means of other chemicals known foracidification, e.g. ammonium sulfate and carbon dioxide. Preferably, theacidification is carried out by means of an acid cation exchanger. Theacidification is preferably carried out by means of a strongly acidcation exchange resin, e.g. a cation exchange resin of sulfonic acidtype. Usually the acidification of the aqueous alkali metal silicatesolution is carried out to provide an acidified aqueous solution, oracid sol, having a pH of from about 1.0 to about 4.0, suitably fromabout 2.0 to about 4.0 and preferably from about 2.2 to about 3.0.

The acid sol may be subjected to further particle growth and/or particleaggregation, or agglomeration. This can be achieved by storage at roomtemperature during somewhat longer periods of time, e.g. for a day or upto about 3 to about 4 days, or by heat treatment in which the period oftime and temperature can be adjusted so that a shorter period of time isused at a higher temperature. It is possible to use a fairly hightemperature during a very short period of time, and it is also possibleto use a lower temperature during a somewhat longer period of time. Inthe heat treatment, the acidified sol can be heated at a temperature ofat least about 25° C., suitably from about 30° C. to about 95° C. andpreferably from about 35° C. to about 80° C. The heat treatment inusually carried out for at least about 10 minutes, suitably from about15 to about 600 minutes or from about 20 to about 240 minutes.

The acid sol is subjected to alkalization, which can be carried outusing conventional alkali, e.g. sodium, potassium and ammonium hydroxideand alkali metal silicate, preferably an aqueous alkali metal silicatesolution or water glass of the starting material as defined above,including its alkali metal, molar ratio, silica content and pH value.The alkalization is suitably carried out to a pH of at least about 10,usually at least about 10.5, and the alkalization is usually carried outto a pH of up to about 11.5. The alkalization is further usually carriedout to a final molar ratio of SiO₂:M₂O, in which M is alkali metal, ofat least about 6:1, suitably from at least about 8:1 and preferably atleast about 10:1, and it can be up to 16:1, suitably up to about 15:1 orup to about 14:1.

Particle aggregation and microgel formation can be achieved in severalways. The degree of aggregate and microgel formation may be affected bythe salt and silica contents during the acidification and alkalizationsteps, and the time period and frequency during which the alkali metalsilicate solution and/or acid sol are passed through a stability minimumat a pH in the range of from about 5 to about 8. By keeping the alkalimetal silicate solution and acid sol under vigorous stirring during thesteps of acidification and alkalization, respectively, at the pH rangeof from 5 to about 8, usually from about 6 to about 8, for a prolongedperiod of time, usually from 1 to 1200 seconds, or from 10 to 600seconds, the desired degree of aggregate and microgel formation can beobtained, corresponding to an S-value of the final silica sol as definedabove. Suitably, the particle aggregation and microgel formation isobtained during the alkalization step at the pH and time period definedabove.

Preferably, the acid sol is alkalized by addition of aqueous alkalimetal silicate solution to obtain an intermediate sol and then theaddition of aqueous alkali metal silicate solution is stopped when theintermediate sol has reached a pH of from about 5 to about 8, preferablya pH from about 6 to about 8. Subsequently, the intermediate sol isstirred for a time period of at least about 10 seconds, suitably atleast about 60 seconds and preferably at least about 300 seconds, and itcan be stirred for a time period of up to about 6000 seconds, suitablyup to about 5000 seconds and preferably up to about 1200 seconds withoutaddition of aqueous alkali metal silicate solution, preferably fromabout 300 to about 1200 seconds, whereupon the intermediate sol isfurther alkalized by addition of aqueous alkali metal silicate solutionto obtain a silica sol having a pH above about 10 and molar ratio ofSiO₂:M₂O, in which M is alkali metal, of from about 6:1 to about 16:1,preferably the pH is from about 10.5 to about 11.5 and the molar ratioof SiO₂:M₂O is from about 10:1 to about 14:1, and preferably M issodium.

The silica sol may comprise aluminum and it may be modified withaluminum. Aluminum modification can be achieved by adding an aluminumcompound to the acid sol, intermediate sol or alkalized sol, i.e. silicasol, for example to the sol being alkalized, i.e. during alkalization,whereby the aluminum compound can be added together with the aqueousalkali metal silicate solution as defined above. Examples of suitablealuminum compounds include alkaline aluminum salts, e.g. aluminates,suitably an aqueous aluminate, e.g. sodium aluminate, potassiumaluminate and mixtures thereof, preferably sodium aluminate. When usingan aluminium compound, it is preferably added in an amount to providethe molar ratio of Si:Al as defined above.

If desired, the silica sol obtained after alkalization can be subjectedto concentration. This can be carried out in known manner, e.g. byosmotic methods, evaporation and ultrafiltration. Concentration of thesilica sol can be made after it has been partly of fully alkalized.

The obtained silica sol can be diluted with water to lower the silicacontent to a desired value. Dilution with water usually takes place tosilica contents within the range of from about 0.05 to about 2% byweight or from about 0.1 to 1.5% by weight. Dilution with purified waterusually increases the stability of the silica sol.

The silica sol of the invention is suitable for use as a flocculatingagent, for example in the production of pulp, paper and board,preferably as a drainage and retention aid, and in water purification,both for purification of different kinds of waste water and forpurification of white water from the pulp and paper industry. The silicasols can be used as a flocculating agent, preferably as a drainage andretention aid, in combination with one or more organic polymers whichcan be selected from anionic, amphoteric, non-ionic and cationicpolymers and mixtures thereof. The use of such polymers as flocculatingagents and as drainage and retention aids is well known in the art.

The polymer can be obtained from natural or synthetic sources, and theycan be linear, branched or cross-linked. Examples of generally suitableorganic polymers include anionic, amphoteric and cationic starches;anionic, amphoteric and cationic acrylamide-based polymers, includingessentially linear, branched and cross-linked anionic and cationicacryl-amide-based polymers; as well as cationic poly(diallyldimethylammonium chloride); cationic polyethylene imines; cationic polyamines;cationic polyamideamines and vinylamide-based polymers,melamine-formaldehyde and urea-formaldehyde resins. Suitably, the silicasol is used in combination with at least one cationic or amphotericpolymer, preferably cationic starch and cationic polyacrylamide, whichcan be used singly, together with each other or together with otherpolymers, e.g. other cationic and/or anionic polymers, suitably anionicpolyacrylamide. The weight average molecular weight of the polymer issuitably above about 1,000,000 and preferably above about 2,000,000. Theupper limit of the weight average molecular weight of the polymer is notcritical; it can be about 50,000,000, usually about 30,000,000 andsuitably about 25,000,000. However, the weight average molecular weightof polymers obtained from natural sources may be higher.

The silica sol of the invention can also be used in combination with oneor more coagulants, either with or without the co-use of the organicpolymer(s) described above. Examples of suitable coagulants includeorganic coagulants, e.g. water-soluble organic polymeric coagulants, andinorganic coagulants. The coagulants can be used singly or together,i.e. a polymeric coagulant can be used in combination with an inorganiccoagulant. The coagulants are preferably cationic. Examples of suitablewater-soluble organic polymeric cationic coagulants include cationicpolyamines, polyamideamines, polyethylene imines, dicyandiamidecondensation polymers and polymers of water soluble ethylenicallyunsaturated monomer or monomer blend which is formed of about 50 to 100mole % of cationic monomer and 0 to about 50 mole % of other monomer.The amount of cationic monomer is usually at least about 80 mole %,suitably 100 mole %. Examples of suitable ethylenically unsaturatedcationic monomers include dialkylaminoalkyl (meth)-acrylates and-acrylamides, preferably in quaternised form, and diallyl dialkylammonium chlorides, e.g. diallyl dimethyl ammonium chloride (DADMAC),preferably homopolymers and copolymers of DADMAC. The organic polymericcationic coagulants usually have a weight average molecular weight inthe range of from 1,000 to 700,000, suitably from 10,000 to 500,000.Examples of suitable inorganic coagulants include aluminum compounds,e.g. alum and polyaluminum compounds, e.g. polyaluminum chlorides,polyaluminum sulphates, polyaluminum silicate sulphates and mixturesthereof, preferably alum and polyaluminum chloride.

Suitable drainage and retention aids for use according to the inventioninclude various combinations of the present silica sol and one or moreorganic polymers, and optional aluminum compounds. Examples of preferreddrainage and retention aids for use in the process of the inventioncomprise (i) silica sol of the invention and cationic starch, (ii)silica sol of the invention, cationic starch and aluminum compound,preferably alum and polyaluminium chloride, (iii) silica sol of theinvention and cationic polyacrylamide, (iv) silica sol of the invention,cationic polyacrylamide and aluminum compound, preferably alum andpolyaluminium chloride, (v) silica sol of the invention, cationic starchand cationic polyacrylamide, (vi) silica sol of the invention, cationicstarch, cationic polyacrylamide and aluminum compound, preferably alumand polyaluminium chloride, (vii) silica sol of the invention, cationicpolyacrylamide and anionic polyacrylamide, (viii) silica sol of theinvention, cationic starch and anionic polyacrylamide, and (ix) silicasol of the invention, cationic starch, anionic polyacrylamide andaluminum compound, preferably alum and polyaluminium chloride.

The one or more drainage and retention aids according to the inventioncan be added to the aqueous cellulosic suspension, or stock, inconventional manner, and in any order. When using drainage and retentionaids comprising silica sol and organic polymer, it is preferred to addthe organic polymer to the stock before adding the silica sol, or to addthem simultaneously, even if the opposite order of addition may be used.In one embodiment, the organic polymer is added before a shear stage,which can be selected from pumping, mixing and cleaning stages, and thesilica sol is added after that shear stage. In another embodiment, thesilica sol and organic polymer are both added late in the process, e.g.after the last stage of high shear, which can be selected from pumping,mixing and cleaning stages, preferably after the centri screen.

When using drainage and retention aids comprising a silica sol andanionic and cationic organic polymers, it is preferred to add thecationic organic polymer to the cellulosic suspension before adding thesilica sol and anionic organic polymer. When using a coagulant, it ispreferably added to the cellulosic suspension before adding the silicasol.

The one or more drainage and retention aids according to the inventionare added to the stock to be dewatered in amounts which can vary withinwide limits depending on, inter alia, type and number of drainage andretention aids, type of furnish, filler content, type of filler, pointof addition, etc. Generally the components are added in amounts thatgive better drainage and retention than is obtained when not adding thecomponents. The silica sol is usually added in an amount of at leastabout 0.001% by weight, often at least about 0.005% by weight,calculated as SiO₂ and based on dry furnish, i.e. dry cellulosic fibersand optional filler, and the upper limit is usually about 1.0% by weightand suitably about 0.5% by weight. Each of the organic polymers isusually added in an amount of at least about 0.001% by weight, often atleast about 0.005% by weight, based on dry furnish, and the upper limitis usually about 3% by weight and suitably about 1.5% by weight. Whenusing a cationic polymeric coagulant, it can be added in an amount of atleast about 0.05% by weight, based on dry furnish. Suitably, the amountis in the range of from about 0.07 to about 0.5% by weight, preferablyin the range from about 0.1 to about 0.35% by weight. When using analuminum compound as the inorganic coagulant, it can be added in anamount of at least about 0.005% by weight, calculated as Al₂O₃ and basedon dry furnish. Suitably the amount is in the range of from about 0.01to about 3.0% by weight, preferably in the range from about 0.05 toabout 2.0% by weight.

Further additives which are conventional in the production of paper andboard can of course be used in combination with the silica sol of theinvention, such as, for example, dry strength agents, wet strengthagents, optical brightening agents, dyes, sizing agents like rosin-basedsizing agents and cellulose-reactive sizing agents, e.g. alkyl andalkenyl ketene dimers and ketene multimers, alkyl and alkenyl succinicanhydrides, etc. The cellulosic suspension can also contain mineralfiller, e.g. kaolin, china clay, titanium dioxide, gypsum, talc andnatural and synthetic calcium carbonates such as chalk, ground marbleand precipitated calcium carbonate.

The term “paper and board”, as used herein, means all types ofcellulosic sheet or web-like products. The process can be used in theproduction of paper and board from different types of suspensions ofcellulosic, or cellulose-containing, fibers and the suspensions suitablycontains at least about 25% by weight and preferably at least about 50%by weight of such fibers, based on dry substance. The suspension can bemade from cellulosic fibers from chemical pulp, e.g. sulphate, sulphiteand organosolv pulps, mechanical pulp, e.g. thermomechanical pulp,chemo-thermomechanical pulp, refiner pulp and groundwood pulp, from bothhardwood and softwood, and can also be based on recycled fibers,optionally from de-inked pulps, and mixtures thereof, preferably thesuspension is made from pulp comprising chemical pulp, recycled fiberpulp and mixtures thereof. Examples of suitable paper and board gradesobtained by the invention include fine paper, test liner, white topliner and craft liner.

The process of this invention is particularly useful in the productionof paper and board from cellulosic suspensions having relatively lowconductivity and/or cationic demand. In this case, the conductivity ofthe cellulosic suspension that is dewatered on the wire is usually atleast about 0.5 mS/cm, suitably at least 0.75 mS/cm, and it is usuallyup to about 5.0 mS/cm, suitably up to about 4.0 mS/cm. Conductivity canbe measured by standard equipment, e.g. a WTW LF 539 instrument suppliedby Christian Berner. Further, in this case, the cationic demand of thecellulosic suspension to which the one or more drainage and retentionaids comprising the silica sol are added is usually below about 0.75mEq./l, or below about 0.5 mEq./l. Cationic demand can be measured by aMütek Particle Charge Detector supplied by BTG.

The pH of the cellulosic suspension can be from about 3 to about 10,suitably at least about 3.5 and preferably from about 4 to about 9.

EXAMPLES

The invention is further illustrated in the following examples which,however, are not intended to limit the same. Parts and % relate to partsby weight and % by weight, respectively, and all suspensions areaqueous, unless otherwise stated.

Example 1

This example illustrates the preparation of a silica sol according tothe invention:

Aqueous sodium silicate solution with a molar ratio of SiO₂:Na₂O of3.5:1 and SiO₂ content of 5.0% by weight was ion exchanged to form anacid silica sol by pumping it through a column filled with cation ionexchange resin Amberlite IR-120 (available from Rohm & Haas) which hadbeen regenerated with sulphuric acid according to manufacturer'sinstruction.

To 3000 g of the obtained acid sol with a SiO₂ content of 5.0% by weightwas added aqueous sodium silicate solution with a molar ratio ofSiO₂:M₂O of about 3.4:1 and SiO₂ content of 5.0% by weight understirring until the mixture obtained reached a pH of 7.7 whereupon theaddition of aqueous sodium silicate solution was temporarily stopped fora time period of 1974 seconds, and then the addition of aqueous sodiumsilicate solution was recommenced until an alkalized sol having a pH of9.8 and molar ratio of SiO₂:Na₂O of 20:1 had been formed.

The alkalized sol was concentrated by ultrafiltration to a SiO₂ contentof 12.2% by weight and then further aqueous sodium silicate solutionhaving a molar ratio of SiO₂:M₂O of 3.4:1 and SiO₂ content of 24.2% byweight was added to form a fully alkalized silica sol having molar ratioof SiO₂:M₂O of about 12. Deionized water was then added to achieve afinal SiO₂ content of about 12% by weight.

The obtained sol of silica particles, designated Ex. 1, had a SiO₂content of 12.1% by weight, pH of 11.0, mole ratio SiO₂:Na₂O of 12.5,concentration of soluble silica of 5200 mg SiO₂/l, S-value of 35% andcontained silica particles with a specific surface area of 870 m²/g.

Example 2

This example illustrates the preparation of another silica sol accordingto the invention:

The procedure of Example 1 was repeated except that the aqueous sodiumsilicate solution was added to the acid sol under stirring until themixture obtained reached a pH of 7.6 whereupon the addition of aqueoussodium silicate solution was temporarily stopped for a time period of4920 seconds, and then the addition of aqueous sodium silicate solutionwas recommenced.

The obtained sol of silica particles, designated Ex. 2, had a SiO₂content of 12.1% by weight, pH of 11.0, mole ratio SiO₂:Na₂O of 12.5,concentration of soluble silica of 5400 mg SiO₂/l, S-value of 33% andcontained silica particles with a specific surface area of 870 m²/g.

Example 3

This example illustrates the preparation of a silica sol according tothe invention:

Sodium silicate solution with a molar ratio of SiO₂:M₂O of 3.4:1 andSiO₂ content of 5.5% was ion exchanged to form an acid silica sol bypumping it through a column filled with strong acid cation exchangeresin Lewatit MDS1368 which had been regenerated with hydrochloric acidaccording to manufacturer's instruction.

To 2000 g of the obtained acid sol with a SiO₂ content of 5.0% was addedaqueous sodium silicate solution with a molar ratio of SiO₂:M₂O of 3.4:1and SiO₂ content of 5.0% under stirring until the mixture obtainedreached a pH of 7.5 whereupon the addition of sodium silicate solutionwas temporarily stopped for a time period of 3290 seconds, and then theaddition of sodium silicate solution was recommenced until an alkalizedsol having a pH of 9.6 had been formed.

The alkalized sol was concentrated by ultrafiltration to a SiO₂ contentof 12%. Sodium silicate solution having a molar ratio of SiO₂:M₂O of3.4:1 and SiO₂ content of 25.2% was diluted with deionized water andadded under stirring to form a fully alkalized silica sol having molarratio of SiO₂:M₂O of about 12:1.

The obtained sol of silica particles, designated Ex. 3, had a SiO₂content of 11.5%, pH of 11.0, molar ratio SiO₂:Na₂O of 12:1,concentration of soluble silica of 3700 mg SiO₂/l, S-value of 36% andcontained silica particles with a specific surface area of 910 m²/g.

Example 4

This example illustrates the preparation of a silica sol according tothe invention:

The procedure of Example 3 was repeated except that the sodium silicatesolution was added to the acid sol under stirring until the mixtureobtained reached a pH of 6.8 whereupon the addition of sodium silicatesolution was temporarily stopped for a time period of 340 seconds, andthen the addition of sodium silicate solution was recommenced.

The obtained sol of silica particles, designated Ex. 4, had a SiO₂content of 11.8%, pH of 11.0, molar ratio SiO₂:Na₂O of 12:1,concentration of soluble silica of 3300 mg SiO₂/l, S-value of 35% andcontained silica particles with a specific surface area of 910 m²/g.

Example 5

This example illustrates the preparation of a silica sol according tothe invention:

An acid silica sol was produced according to the procedure of Example 3.To 2000 g of the acid sol with a SiO₂ content of 5.21% was added 70 g ofaqueous sodium silicate solution with a molar ratio of SiO₂:Na₂O of3.4:1 and SiO₂ content of 5.0% under stirring whereupon the addition ofsodium silicate solution was temporarily stopped for a time period of310 seconds, and then the addition of another 351 g of the sodiumsilicate solution was recommenced until an alkalized sol having a pH of9.6 had been formed. To the alkalized sol was added 43.9 g of sodiumaluminate solution with Al₂O₃ content of 2.43% and Na₂O content of 1.84%under stirring to form an aluminized sol.

The aluminized sol was concentrated by ultrafiltration to a SiO₂ contentof 13.1%. To 798 g of the obtained sol was added 56.5 g of sodiumsilicate solution having molar ratio of SiO₂:Na₂O of 3.4 and SiO₂content of 25.2% and under stirring to form an alkalized silica sol.

The obtained sol of silica particles, designated Ex. 5, had a SiO₂content of 12.5%, pH of 10.9, molar ratio SiO₂: Na₂O of 11.8:1, molarratio Si:Al of 76:1, concentration of soluble silica of 5900 mg SiO₂/l,S-value of 28% and contained silica particles with a specific surfacearea of 930 m²/g.

Example 6

This example illustrates the preparation of a silica sol according tothe invention:

The procedure of Example 1 was repeated except that small modificationswere made to the pH and time period for the temporary stop of additionof aqueous sodium silicate solution in the alkalization step, and to thefurther addition of aqueous sodium silicate solution and water to thealkalized silica sol.

The obtained sol of silica particles, designated Ex. 6, had a SiO₂content of 12.2%, pH of 11.0, molar ratio SiO₂:Na₂O of 12.6:1,concentration of soluble silica of 4900 mg SiO₂/l, S-value of 38% andcontained silica particles with a specific surface area of 920 m²/g.

Example 7

This example illustrates the preparation of a prior art silica sol usedfor comparison:

The procedure of Example 1 was repeated except that the aqueous sodiumsilicate solution was continuously added to the acid sol under stirring,i.e. without any temporary stop of addition in the pH range of from 5 to8, until an alkalized sol having a pH of 9.9 and molar ratio SiO₂:Na₂Oof 20:1 had been formed.

The obtained sol of silica particles, designated Ref. 1, had a SiO₂content of 11.0% by weight, pH of 11.1, mole ratio SiO₂:Na₂O of 11.7,concentration of soluble silica of 6900 mg SiO₂/l, S-value of 47% andcontained silica particles with a specific surface area of 900 m²/g.

Example 8

A silica sol was prepared according to the general disclosure of U.S.Pat. No. 5,368,833. The obtained sol of silica particles, designatedRef. 2, had a SiO₂ content of 7.7% by weight, pH of 9.6, mole ratioSiO₂:Na₂O of 31, mole ratio Si:Al of 44, concentration of soluble silicaof 1800 mg SiO₂/l, S-value of 29% and contained silica particles with aspecific surface area of 750 m²/g.

Example 9

A silica sol was prepared according to the general disclosure of WO00/66491. The obtained silica sol, designated Ref. 3, had a SiO₂ contentof 14.8% by weight, pH of 10.8, molar ratio SiO₂:Na₂O of 20:1,concentration of soluble silica of 2100 mg SiO₂/l, and S-value of 36%,and contained silica particles with a specific surface area of 740 m²/g.

Example 10

A silica sol was prepared according to the procedure of Example 4 of WO00/66491. The obtained sol of silica particles, designated Ref. 4, had aconcentration of soluble silica of 1700 mg SiO₂/l.

Example 11

A silica sol was prepared according to the procedure of Example 1 of US2005/228057 A1. The obtained sol of silica particles, designated Ref. 5,had a molar ratio Si:Al of 26.2:1, and concentration of soluble silicaof 1700 mg SiO₂/l.

Example 12

A silica sol was prepared according to the procedure of Example 4 of US2005/228057 A1. The obtained sol of silica particles, designated Ref. 6,had a molar ratio Si:Al of 30.2:1, concentration of soluble silica of2600 mg SiO₂/l.

Example 13

A silica sol was prepared according to the general disclosure of WO2008/150230. The obtained sol of silica particles, designated Ref. 7,had a SiO₂ content of 6.4%, pH of 8.3, molar ratio SiO₂:Na₂O of 16.9:1,molar ratio Si:Al of 8.5:1, concentration of soluble silica of 2700 mgSiO₂/l, S-value of 12% and contained silica particles with a specificsurface area of 1060 m²/g.

Example 14

Retention performance was evaluated by means of a Britt Dynamic DrainageJar. The cellulosic suspension used was based on a blend of 30% byweight bleached birch sulphate, 30% by weight bleached pine sulphate and40% by weight ground calcium carbonate. Stock volume was 500 ml,consistency 0.45% and pH about 8.4. Conductivity of the stock wasadjusted to 1.48 mS/cm by addition of 0.1 g/l calcium chloride and 1.8g/l sodium sulphate.

In the tests, silica sols (SiO₂) were used in conjunction with apolyaluminium chloride (PAC), cationic starch (CS) having a degree ofcationic substitution of 0.042, cationic polyacrylamide (CPAM) being acopolymer of acrylamide (90 mole %) and dimethylaminoethyl acrylatemethyl chloride quaternary salt (DMAEA-MCQ) (10 mole %) and having aweight average molecular weight of about 10 million Dalton.

The cellulosic suspension was stirred in a baffled jar at a constantspeed of 1200 rpm throughout the test and chemical additions wereconducted as follows:

-   -   adding PAC to the cellulosic suspension 40 seconds prior to        dewatering,    -   adding CS to the cellulosic suspension 35 seconds prior to        dewatering,    -   adding CPAM to the cellulosic suspension 20 seconds prior to        dewatering,    -   adding SiO₂ to the cellulosic suspension 10 seconds prior to        dewatering, and    -   dewatering the cellulosic suspension and then measuring the        turbidity of the filtrate diluted 15 times with deionized water.

The retention performance of the silica sols is shown in Table 1, inwhich addition levels are calculated as dry product on dry furnish:

TABLE 1 Test PAC CS CPAM SiO₂ Turbidity No. [kg/t] [kg/t] [kg/t] [type][kg/t] [NTU] 1 — — — — — 319 2 1 8 0.3 — — 139 3 1 8 0.3 Ex. 1 0.6 90 41 8 0.3 Ref. 1 0.6 105 5 1 8 0.3 Ref. 2 0.6 113 6 1 8 0.3 Ref. 3 0.6 101

As is evident from Table 1, the silica sol according to the invention,Ex. 1, showed significantly better retention performance over the silicasols used for comparison, Ref. 1, Ref. 2, and Ref. 3.

Example 15

Drainage performance was evaluated by means of a Dynamic DrainageAnalyser (DDA), available from PulpEye AB, Sweden, which measures thetime for draining a set volume of stock. The stock was stirred in abaffled jar at a speed of 1500 rpm throughout the test while additionsof chemicals were made. A stock volume of 800 ml was drained through awire when removing a plug and applying vacuum to that side of the wireopposite to the side on which the stock is present. Drainage performanceis reported as the dewatering time (s).

The stock used in this Example was based on a furnish containing 60%chemical pulp, which consisted of 80% hardwood and 20% softwood, and 40%filler, precipitated calcium carbonate (PCC). Salts were added toincrease the conductivity to about 1.5 mS/cm, pH was about 8.4 and thepulp consistency was about 5.4 g/l.

In the tests, the chemical additives of Example 14 were used except thatno CPAM was used. The cellulosic suspension was stirred in the baffledjar and chemical additions were made as follows:

-   -   adding PAC to the cellulosic suspension 43 seconds prior to        dewatering,    -   adding CS to the cellulosic suspension 35 seconds prior to        dewatering,    -   adding SiO₂ to the cellulosic suspension 10 seconds prior to        dewatering, and    -   dewatering the cellulosic suspension while automatically        recording the dewatering time.

Table 2 shows the drainage performance of the silica sols, in which theaddition levels are calculated as dry product on dry furnish:

TABLE 2 Test PAC CS SiO₂ Dewatering No. [kg/t] [kg/t] [type] [kg/t][seconds] 1 0.13 7 — — 21.8 2 0.13 7 Ex. 3 0.15 16.4 3 0.13 7 Ex. 3 0.312.5 4 0.13 7 Ex. 3 0.6 8.7 5 0.13 7 Ex. 3 1.0 6.8 6 0.13 7 Ex. 4 0.1516.4 7 0.13 7 Ex. 4 0.3 12.4 8 0.13 7 Ex. 4 0.6 8.6 9 0.13 7 Ex. 4 1.06.7 10 0.13 7 Ref. 4 0.15 18.6 11 0.13 7 Ref. 4 0.3 15.7 12 0.13 7 Ref.4 0.6 11.6 13 0.13 7 Ref. 4 1.0 8.7 14 0.13 7 Ref. 5 0.15 17.9 15 0.13 7Ref. 5 0.3 13.7 16 0.13 7 Ref. 5 0.6 9.6 17 0.13 7 Ref. 5 1.0 7.5 180.13 7 Ref. 6 0.15 17.5 19 0.13 7 Ref. 6 0.3 13.4 20 0.13 7 Ref. 6 0.69.1 21 0.13 7 Ref. 6 1.0 7.2

As is evident from Table 2, the silica sols according to the invention,Ex. 3 and Ex. 4, showed significantly better drainage performance overthe silica sols used for comparison, Ref. 4, Ref. 5, and Ref. 6.

Example 16

Drainage performance was evaluated according to the procedure of Example15 using different silica sols. The stock used in this Example was basedon a furnish containing about 60% chemical pulp, which consisted ofabout 80% hardwood and about 20% softwood, and about 40% filler,precipitated calcium carbonate (PCC). Salts were added to increase theconductivity to about 1.5 mS/cm, pH was about 8.3 and the pulpconsistency was about 5.4 g/l.

Table 3 shows the drainage performance of the silica sols, in which theaddition levels are calculated as dry product on dry furnish:

TABLE 3 Test PAC CS SiO₂ Dewatering No. [kg/t] [kg/t] [type] [kg/t][seconds] 1 0.13 7 — — 24.3 2 0.13 7 Ex. 5 0.3 14.7 3 0.13 7 Ex. 5 0.610.0 4 0.13 7 Ex. 5 0.9 8.0 5 0.13 7 Ex. 6 0.3 15.7 6 0.13 7 Ex. 6 0.610.7 7 0.13 7 Ex. 6 0.9 8.6 8 0.13 7 Ref. 5 0.3 18.4 9 0.13 7 Ref. 5 0.614.1 10 0.13 7 Ref. 5 0.9 11.1 11 0.13 7 Ref. 6 0.3 17.2 12 0.13 7 Ref.6 0.6 12.0 13 0.13 7 Ref. 6 0.9 9.5 14 0.13 7 Ref. 7 0.3 16.6 15 0.13 7Ref. 7 0.6 12.0 16 0.13 7 Ref. 7 0.9 10.0

As is evident from Table 3, the silica sol containing aluminium-modifiedsilica particles according to the invention, Ex. 5, showed betterdrainage performance over the silica sols containing aluminium-modifiedsilica particles used for comparison, Ref. 5, Ref. 6 and Ref. 7. Alsothe silica sol Ex. 6 according to the invention showed better drainageperformance over the silica sols used for comparison.

1.-20. (canceled)
 21. A process for producing the silica sol, theprocess comprising, (a) acidifying an aqueous alkali metal silicatesolution to a pH of from 1 to 4 to form an acid sol, (b) alkalizing theacid sol by addition of aqueous alkali metal silicate solution to obtainan intermediate sol and interrupt the addition of aqueous alkali metalsilicate solution when the intermediate sol has reached a pH of fromabout 5 to about 8, (c) stirring the intermediate sol for a time periodof from about 10 to about 6000 seconds without addition of aqueousalkali metal silicate solution, and (d) alkalizing the intermediate solby addition of aqueous alkali metal silicate solution to obtain a silicasol having a pH above about 10 and molar ratio of SiO₂:M₂O, in which Mis alkali metal, of from about 6:1 to about 16:1, (e) optionally addingan aluminum compound to the acid sol, intermediate sol or silica sol.wherein the concentration of soluble silica in the sol is above about3000 mg SiO₂/l, and the sol has an S-value of from about 18 to about40%.
 22. The process according to claim 21, wherein the processcomprises adding an aluminum compound to the acid sol, intermediate solor silica sol.
 23. The process according to claim 21, wherein thealuminum compound is added in an amount to obtain a silica sol having amolar ratio of Si:Al of from about 25:1 to about 250:1.
 24. A silica solobtained by the process according to claim
 21. 25. A method offlocculation, comprising providing the silica sol according to claim 1as a flocculating agent.
 26. A process for producing paper and boardcomprising, (iv) providing an aqueous suspension comprising cellulosicfibers; (v) adding to the suspension one or more drainage and retentionaids comprising a silica sol according to claim 1; and (vi) dewateringthe obtained suspension to provide a sheet or web of paper board. 27.The process according to claim 26, wherein said one or more drainage andretention aids comprise a cationic polymer.
 28. The process according toclaim 26, wherein said one or more drainage and retention aids comprisean anionic polymer.
 29. The process according to claim 26, furthercomprising adding to the suspension an aluminum compound.